This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-397297, filed Dec. 27, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to a cathode-ray tube apparatus and more particularly to a color cathode-ray tube apparatus capable of improving an oval distortion of a beam spot on a peripheral portion of a phosphor screen and displaying an image with high quality.
2. Description of the Related Art
In general terms, a color cathode-ray tube (CRT) apparatus comprises an inline electron gun assembly for emitting three electron beams, which are horizontally arranged in line, and a deflection yoke for generating non-uniform deflection magnetic fields for horizontally and vertically deflecting the three electron beams. As an electron gun assembly emitting three electron beams, there is known a QPF (Quadru-Potential Focus) double-focus type electron gun assembly, which comprises, as shown in FIG. 8, three in-line cathodes K, and six grids G1 to G6 successively arranged toward a phosphor screen. Each of the grids G1 to G6 has three electron beam passage holes in association with the three in-line cathodes K.
In this electron gun assembly, a voltage of about 150V is applied to the cathodes K. The first grid G1 is grounded. The second grid G2 is connected to the fourth grid G4 within the tube and supplied with a voltage of about 600V. The third grid G3 is connected to a first segment G5-1 of the fifth grid G5 within the tube and supplied with a focus voltage of about 6 KV. A second segment G5-2 of the fifth grid G5 is supplied with a dynamic focus voltage obtained by superimposing upon a reference voltage of about 6 KV an AC component increasing in accordance with an increase in degree of deflection of electron beams. The sixth grid G6 is supplied with an anode voltage of about 26 KV.
An electron beam generating section is constituted by the cathodes K, first grid G1 and second grid G2 and generates electron beams. A prefocus lens is constituted by the second grid G2 and third grid G3 and prefocuses the electron beams emitted from the electron beam generating section. A sub-lens is constituted by the third grid G3, fourth grid G4 and first segment G5-1 and further prefocuses the electron beams. A main lens is constituted by the second segment G5-2 and sixth grid G6 and ultimately focuses the electron beams on the phosphor screen.
In a non-deflection mode in which electron beams are focused on a central portion on the phosphor screen, the electron beams generated from the electron beam generating section are focused on the phosphor screen by the prefocus lens, sub-lens and main lens. At this time, since there is no potential difference between the first segment G5-1 and second segment G5-2, a quadrupole lens is not created.
On the other hand, in a deflection mode in which electron beams are deflected onto a peripheral portion of the phosphor screen, a higher voltage is applied to the second segment G5-2 and a potential difference occurs between the first segment G5-1 and second segment G5-2. Thus, a quadrupole lens is created. The quadrupole lens created at this time has such an astigmatism that it has a horizontal focusing function and a vertical diverging function. At the same time, a potential difference between the second segment G5-2 and sixth grid G6 decreases, and the lens power of the main lens lowers. Thereby, a focus error due to an increase in distance over which the electron beams reach the phosphor screen is corrected, and a deflection aberration due to non-uniform magnetic fields is compensated.
In order to enhance the image quality of the color CRT apparatus, it is necessary to improve focus characteristics on the phosphor screen. In particular, in the case of a color CRT apparatus emitting three in-line electron beams, a beam spot on the phosphor screen may disadvantageously have an elliptically deformed core and blur portion due to deflection aberration, as shown in FIG. 9A.
In a general double-focus electron gun assembly, a low-voltage side electrode constituting the main lens is composed of a plurality of grids such as the first segment G5-1 and second segment G5-2. A quadrupole lens is created between these segments in accordance with deflection of the electron beam. Thereby, deflection aberration is compensated, and the problem of blur is improved, as shown in FIG. 9B.
However, as shown in FIG. 9B, oval deformation of the beam spot still remains at an end portion in a horizontal axis H and an end portion in a diagonal axis D on the phosphor screen. The reason is as follows. If the sub-lens, quadrupole lens, main lens and deflection aberration components included in deflection magnetic fields are assumed to be a single lens, the horizontal lens magnification increases and the vertical lens magnification decreases. This results in such oval deformation. Consequently, the vertical dimension of the beam spot becomes too small, and a moire may occur due to interference with the shadow mask. A character, etc., if formed with such a beam spot, could not easily be viewed.
To solve this problem, an electron gun assembly with a double-quadrupole lens structure is proposed. As is shown in FIG. 10, the double-quadrupole lens structure has the same basic structure as shown in FIG. 8. The third grid G3 comprises a first segment G3-1 and a second segment G3-2. The second segment G3-2 is connected to the second segment G5-2 and supplied with a dynamic focus voltage at the time of deflection.
In the deflection mode, a first quadrupole lens dynamically varying in synchronism with deflection magnetic fields is created between the first segment G3-1 and second segment G3-2. The first quadrupole lens has a horizontal diverging function and a vertical focusing function. In short, the first quadrupole lens has an astigmatism with polarities opposite to those of a second quadrupole lens created between the first segment G5-1 and G5-2.
Thereby, if the first quadrupole lens, sub-lens, second quadrupole lens, main lens and deflection aberration components included in deflection magnetic fields are assumed to be a single lens, a difference between the horizontal lens magnification and the vertical lens magnification can be decreased and the oval deformation of the beam spot improved.
Compared to the conventional double-focus electron gun assembly, this electron gun assembly with the double-quadrupole lens structure requires quadrupole lenses with higher power. In particular, the diameter of each electron beam passing through the first quadrupole lens is small. Thus, in order to obtain sufficient effect of improving oval deformation, the first quadrupole lens must have a very high lens power.
The quadrupole lens is formed by arranging a pair of grids, as shown in FIGS. 12A and 12B, such that their electron beam passage holes are opposed to each other. The electron beam passage holes in one of the grids are horizontally elongated ones, and the electron beam passage holes in the other grid are vertically elongated ones. However, a necessary lens power may not be obtained in order to sufficiently improve the oval deformation of the beam spot.
This phenomenon will now be described referring to FIGS. 13A and 13B. In FIGS. 13A and 13B, an electron beam enters from the left side in the Figures, and exits to the right side. Assume that a voltage V1 is applied to an incidence-side grid Gin, a voltage V2 is applied to an emission-side grid Gout, and V1 less than V2.
As regards the vertical direction, as shown in FIG. 13A, the electron beam entering the quadrupole lens suffers a strong focusing action since the vertical dimension of the incidence-side grid Gin is small. The electron beam exiting the quadrupole lens suffers a weak diverging action since the vertical dimension of the emission-side grid Gout is large. As a result, the quadrupole lens provides a relative focusing action upon the electron beam in the vertical direction.
On the other hand, as regards the horizontal direction, as shown in FIG. 13B, the electron beam entering the quadrupole lens suffers a weak focusing action since the horizontal dimension of the incidence-side grid Gin is large. The electron beam exiting the quadrupole lens suffers a strong diverging action since the horizontal dimension of the emission-side grid Gout is small. As a result, the quadrupole lens provides a relative diverging action upon the electron beam in the horizontal direction.
In the quadrupole lens having this structure, the incidence-side lens action and emission-side lens action contradict each other, and part of the lens actions is canceled. Thus, strong lens power cannot be obtained.
In a method of solving this problem, a pair of grids as shown in FIGS. 11A and 11B are arranged such that their electron beam passage holes are opposed to each other. One of the grids has screen-like projection portions at its horizontal end portions of the electron beam passage holes, and the other grid has screen-like projection portions at its vertical end portions of the electron beam passage holes.
According to this method, the lens space can be extended in the direction of travel of electron beams, and the time, over which the electron beams suffer the lens action of the quadrupole lens, can be increased. Thus, a strong lens power can be obtained.
However, in consideration of productivity, the formation of the screen-like projection portions requires cutting and bending machining by a pressing process. The dimensional precision obtained by this machining is lower than that obtained by a punching process. It is difficult to enhance the dimensional precision for forming the screen-like projection portions. This leads to variance or degradation in focusing performance, such as variance in the quadrupole lens power or undesirable deflection of electron beams. Moreover, since the grids have projections in the direction of travel of electron beams, the dimension of the grids in this direction cannot be decreased and the degree of freedom is limited in designing the electron gun assembly. Besides, the number of parts increases and consequently the manufacturing cost rises.
The present invention aims at solving the above problems, and the object of the invention is to provide a cathode-ray tube apparatus capable of having stable and good focusing characteristics over the entire phosphor screen.
According to an aspect of the invention, there is provided a cathode-ray tube apparatus comprising:
an electron gun assembly including an electron beam generating section which generates electron beams, and a main lens which focuses the electron beams generated by the electron beam generating section on a phosphor screen; and
a deflection yoke which generates deflection magnetic fields that horizontally and vertically deflect the electron beams emitted from the electron gun assembly,
wherein the electron gun assembly includes at least one multi-polar lens that is created in accordance with deflection of the electron beams,
the at least one multi-polar lens is created by two mutually opposing grids,
the two mutually opposing grids have non-circular electron beam passage holes in their mutually facing surfaces, and
each of the electron beam passage holes has a waist portion which minimizes a horizontal or vertical dimension of a region for passing the associated electron beam.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.